Mail will get a number of longed-for improvements in iPhone OS 4.0. Apple has finally added a unified inbox to Mail, so those of you setting up multiple e-mail accounts on your iPhone can see all your mail at once without switching accounts. Business users will likely applaud the ability to add more than one Exchange account. And Apple made switching between different inboxes much faster.
Other improvements include a threaded message view, which keeps related replies together, and the ability to open mail attachments if you have an app installed that can handle a particular file type.